If you work in landscape architecture and design you’ve probably heard about rain gardens being specified in developments with increasing frequency. But what are they and what do you need to know about them?
What are rain gardens?
Rain gardens are landscaped depressions that filter and attenuate water. They use plants and formulated soil compositions to hold and slow water during storms and intense weather events. By slowing the movement of water, rain gardens relieve stress on nearby drainage infrastructure and water courses like rivers and streams. These engineered natural solution have a host of additional benefits as well as managing excess water, they can:
- Create habitats for urban biodiversity
- Clean water before it enters drainage systems
- Sequester carbon, helping mitigate change
- Purify air and reduce the urban heat island effect
Why use rain gardens?
Rain gardens are becoming increasingly popular in sustainable design and water management for a number of reasons:
- Reduced flood risk – through attenuation and water storage, this is particularly important in the UK where sewage and water drainage use the same systems.
- Improved ground stability – plants with long root systems protect against soil erosion.
- Water filtration – organic matter, plants, and biochar filter toxins out of stormwater, helping to reduce pollution entering water courses and drains.
- Providing habitat – rain gardens can become homes for pollinators and small wildlife.
- Supporting local aquifers – by allowing water to soak into the ground.
- Improved air quality – plant life removes carbon and pollution from the air via respiration.
- Physical and mental health benefits – plant life and green spaces improve physical, emotional and mental health in urban communities.
- Carbon sequestering – plants and organic matter hold carbon and protect against climate change.
- Cooling – greenery can reduce ambient temperatures and combat the urban heat island effect.
What do I need to know about rain gardens?
Filtration vs infiltration
Rain gardens are separated into 2 categories:
Infiltration rain gardens pass water through layers of mulch, soil and root systems before slowly permeating into adjacent native soil and aquifers.
Filtration rain gardens work in much the same way but contain a gravel or rock layer separated from the soil by a permeable geotextile membrane. A porous pipe embedded in the rock layer directs the filtered water from the rain garden to a storm system or water course. Filtration systems are more common in urban environments due to poor draining and compacted soils and contamination concerns.
What are rain gardens made from?
Rain gardens combine natural and structural elements to manage and filter water effectively.
Amended soil – a blend of sand, topsoil and compost or organic material for infiltration and filtration. Biochar is popular for pollution filtration, beneficial microbial activity, as well as its function as a long-term carbon sink.
Mulch layer – rain gardens feature a top layer of mulch (often wood chips) to trap moisture in the soil, suppress weed growth as well as maintain stable soil temperature. As the mulch breaks down it adds nutrients to the soil, helping with the ongoing life cycle and fertility of the rain garden ecosystem.
Plants – deep rooted plants that are tolerant to both wet and dry conditions. Root systems improve water infiltration and help remove pollutants as well as increase soil stability. Moisture levels in a rain garden are always in flux so it’s important that plants that can tolerate both wet and dry conditions. Plants that prefer wet conditions are placed in the centre of the garden where the soil is wettest, while grasses and dry tolerant plants are often arranged on the edge. Additionally perennials can be used to create habitats for pollinators like bees and butterflies, creating a self-sufficient ecosystem.
Berm – a small, raised barrier on the side of the rain garden acts like a bowl, holding water so it can permeate through the system, without spilling onto surrounding areas.
Overflow structure – an outlet allows water to leave the rain garden when needed, preventing flooding and damage, or to direct water to specified drainage areas in filtration systems.
Geotextiles – durable permeable fabrics (often non-woven for filtration and permeability) are used to separate layers within the rain garden and provide structural stability. Sometimes geotextiles can be used as liners to prevent infiltration and keep water within the rain garden system.
Drainage layer of rock – in planters and filtration systems, a base layer of rock or gravel is used for storage and quick drainage. Voids between rocks attenuate and hold water, a geotextile layer is used to keep soil and rock layers separate.
Reservoirs – an optional subsurface zone that can complement or replace a gravel reservoir. Geocellular crates used in stormwater management schemes can be used for superior water storage. A wicking mechanism can be used to provide passive irrigation, allowing water to travel back into the rain garden, helping maintain consistent moisture levels. Use of biochar and amended soil with capillary properties can enhance the wicking effect.
Sediment traps – by trapping soil and pollutants at rainwater ingress points sediment traps improve water quality – they may need to be cleaned periodically.
Rain garden design considerations
Placement
Rain gardens should be at least 3 metres from building foundations to avoid water damage, and gently sloping sites are preferable to avoid water ponding.
Sizing
Rain garden area should be about 20% of the surface area it receives runoff from, e.g. road or roof area.
Typical depths in rain gardens are between 10 – 30cm, however this depends on their location.
Drainage
Soil should drain at a rate of approximately 5cm per hour for effective infiltration. An overflow outlet or spillway should be used to manage excess water.
Structural concerns
Berms should be used on the downhill edge to hold water and prevent erosion. Splash pads can be used to reduce soil displacement. Geotextiles should be used to reduce clogging of drainage layers.
Maintenance
Design your rain garden for easy access for maintenance, including removing debris, re mulching and protecting plant health for a robust and long lasting system.
Rain gardens – some success stories from around the UK
Royal Docks rain garden
- Project includes over 50 varieties of trees and shrubs – including trees up to 6 metres tall – chosen for durability, to improve the biodiversity, manage pollution and air quality, as well as make people feel safer and more comfortable in urban spaces.
Esther Road rain garden, Waltham Forest
- An innovative project that repurposed urban parking space to improve flood resilience as well as promote biodiversity and green public spaces.
- 83m2 of rain gardens were created with a variety of over 550 drought tolerant plants.
- Rain garden systems incorporated a stone layer with 30% void space to serve as a reservoir for water storage. In heavy weather the stone layer fills first, with excess water overflowing into the sewer system.
- The project incorporated SuperDrain a porous asphalt surface that allows water to drain directly through the carriageway.
- Successful SuDS project – proving that small scale suds interventions can have large scale implications in creating a sustainable and climate resilient future.
Renfrew Close, London
- A demonstration project to show how SuDS systems can be successfully retrofitted into existing developments.
- Project consisting of four rain garden basins located within the central communal grass area of surrounding social housing blocks.
- Rain gardens fed water from roof downpipes and through channels at ground level and under pathways.
- The rain garden basins were linked so excess water can move between them and prevent overflow.
- Residential interaction was encouraged via implementation of stepping stones, balance beams and boulders throughout the rain gardens.
- A biodiverse and sustainable flood prevention and drainage system that enhances community engagement and improves communal outdoor space.
Read more about the built environment on the Foundations blog: